Prior art CO.sub.2 absorbent systems generally fall into one of two categories: (1) amines or (2) aqueous salt solutions.
A variety of amines, mostly alkanolamines, are used as CO.sub.2 absorbents; see Hydrocarbon Processing, 57-80 (April 1988). Primary and secondary amines, such as monoethanolamine (MEA) and diethanolamine (DEA), react with CO.sub.2 to form carbamates in accordance with the following reaction: EQU 2RNH.sub.2 +CO.sub.2 .revreaction.RNHCO.sub.2.sup.- +RNH.sub.3.sup.+
Since carbamates are relatively stable, heating is required to regenerate the CO.sub.2 -free absorbent. Typical heats of reaction of amines with CO.sub.2 are relatively high, 10-20 kcal/mole CO.sub.2. Tertiary amines, such as triethanolamine (TEA) and methyldiethanolamine (MDEA), cannot form carbamates. Such amines generate hydroxide in aqueous solutions and hydroxide subsequently reacts with CO.sub.2 to give HCO.sub.3.sup.- and/or CO.sub.3.sup.2-. The heats of reaction of CO.sub.2 with tertiary amines are less than those of primary and secondary amines and less energy is needed to desorb CO.sub.2 from tertiary amine solutions.
Recently, the use of sterically hindered amines as CO.sub.2 absorbents was reported by Nirula, S. C.; Ashraf, M. SRI International Report No. 180 (1987). Due to steric hinderance at the amine nitrogen, the resulting carbamate is less stable than that of an unhindered amine, resulting in absorption capacities and rates greater than those of conventional amines. Conventional amines have capacities of only 0.5 mole CO.sub.2 /mole amine except at quite high pressures. Two moles of amine are necessary to form one mole of carbamate. Since the carbamate is stable, hydrolysis to HCO.sub.3.sup.- and protonated amine does not occur readily. By contrast, the unstable carbamate of a hindered amine is readily hydrolyzed and a maximum capacity of 1.0 mole CO.sub.2 /mole amine can be obtained.
The absorption of CO.sub.2 using aqueous solutions of strongly alkaline salts has been practiced for over 50 years and numerous reviews of the subject are available, see Encyclopedia of Chemical Processing and Design McKetta, J. J. Editor Vol. 6, p 292-310 (1978). The most commonly used salts are sodium and potassium carbonate, phosphate, borate, arsenite, and phenolate. The common use of potassium carbonate at relatively high temperatures for both absorption and desorption led to the hot potassium carbonate or hot pot process (HPC) in the early 1950s. The addition of activators or catalysts to enhance the rates of absorption and desorption and of corrosion inhibitors followed.
Several processes have been developed which use aqueous potassium carbonate with added activators (e.g. alkanolamines) and corrosion inhibitors. In one well known process, the Giammarco-Vetrocoke process, activators such as glycine, arsenic trioxide, and selenous acid are used. The Alkazid process uses aqueous solutions of potassium methylaminopropionate or, in some cases, sodium phenolate. The SRI International report cited above also teaches the use of sterically hindered amines as activators for potassium carbonate solutions. In all of the above aqueous salt processes, a strongly alkaline salt results in a high concentration of hydroxide. Reaction of CO.sub.2 with hydroxide leads to bicarbonate and/or carbonate. CO.sub.2 is desorbed by decomposition of bicarbonate which, at high hydroxide concentrations, requires heating.
Processes using strongly alkaline amino acid salts have also been reported. Amino acid salts can also react with CO.sub.2 to form carbamates. Guyer and Purner, Helv. Chim. Acta 21, 1337-1345 (1938), evaluated solutions of the sodium and potassium salts of glycine and alanine as CO.sub.2 absorbents. CO.sub.2 desorption required heating. U.S. Pat. No. 3,042,483 describes the use of concentrated aqueous solutions of salts of amino acids as CO.sub.2 absorbents. In particular, salts of taurine and substituted taurines, glycine, alanine, and sarcosine are claimed as absorbents. Each absorbent is either a primary or secondary amine capable of carbamate formation. Heat is required for desorption. German Pat. No. 2,605,618 describes the use of aqueous solutions of potassium methylaminopropionate to absorb CO.sub.2 from air. The absorbent is regenerated by steam stripping. South African Pat. No. 7,603,420 reports the use of aqueous solutions of alkali metal salts of N-dialkylaminomonocarboxylic acids as CO.sub.2 absorbents between 20.degree. and 60.degree. C. CO.sub.2 is removed under reduced pressure and, optionally, heating. Japanese Pat. No. 61-101244 describes the preparation of an absorbent consisting of an alkali metal salt of N-methylalanine support on a porous material (e.g. alumina). The supported salt is capable of absorbing CO.sub.2 in closed spaces, apparently via hydrolysis and formation of K.sub.2 CO.sub.3. The absorbent is regenerated by heating up to 200.degree. C. with no reported salt decomposition.
A number of reports have been written describing the solubility of CO.sub.2 in aqueous solutions of weakly alkaline salts. The solubility of CO.sub.2 in aqueous solutions of the sodium salts of succinic, oxalic, and malonic acids, potassium chromate, potassium fluoride, borax, ammonium molybdate, and others were determined to obtain equilibrium constants of weak acids. As reported in Solubilities of Inorganic and Metal-Organic Compounds, Linke, W. F. Editor, Vol. I (1958), the CO.sub.2 solubilities are somewhat greater than those of aqueous solutions containing neutral salts. For example, at 30.degree. C. the solubility of CO.sub.2 in 1.19M KF is 0.14M versus that in water, 0.032M. Fr. Pat. No. 1130145 describes the use of aqueous solutions of K.sub.2 HPO.sub.4 to absorb CO.sub.2 and H.sub.2 S. The absorbent was regenerated by blowing with an inert gas or by application of a slight vacuum. Fr. Pat. No. 1135262 describes the use of weakly basic salt solutions as absorbents. A salt containing the anion, A, of a weak acid is used and the following reaction occurs: EQU NaA+CO.sub.2 +H.sub.2 O.revreaction.AH+NaHCO.sub.3
1.5M solutions of Na.sub.2 HPO.sub.4 and Na.sub.2 CrO.sub.4 are cited as examples. Desorption of CO.sub.2 does not require heating. The first addition to this patent, addition No. 68,830 describes the use of 1.5M aqueous solutions of alkali metal and ethanolamine salts containing various anions, among them, sulfite, malate, succinate, and malonate. The best results were obtained for acids which results in pH of 4-5 and salts with pH of 9-10. In the second addition to the above patent, addition No. 71,112, a variation using aqueous potassium chromate is described where slight heating (to 50.degree. C.) is used for desorption. The above patents and additions also appeared as Brit. Pat. No. 831532.
Ger. Pat. No. 1177619 describes the use of aqueous solutions of alkali salts of heteropoly acids, where one is a weak acid and the other is a weak to medium strong acid, as CO.sub.2 absorbents. The absorbent can be regenerated by treating with air at 30.degree. C.
The kinetics of the reaction of CO.sub.2 with aqueous Na.sub.2 CrO.sub.4 has been investigated by R. Vaikunrm, et al. Ind. J. Tech. 16 379-383 (1978). The reaction generates HCO.sub.3.sup.- and Cr.sub.2 O.sub.7.sup.2- and is second order in chromate. Belton et al. J. Inorg. Nucl. Chem. 43, 614-615 (1981) reported that the reaction of CO.sub.2 with solid Na.sub.3 PO.sub.4 .cndot.11H.sub.2 O results in the apparently irreversible formation of Na.sub.2 HPO.sub.4 .cndot.7H.sub.2 O and NaHCO.sub.3.
U.S. Pat. Nos. 4,235,607 and 4,239,510.sup.27 describe a method for purification of natural gas by removal of CO.sub.2. The absorbent is sea water and the separation apparatus is constructed near the ocean floor. CO.sub.2 rich sea water is simply discharge without regeneration.
U.S. Pat. No. 4,472,283 reports that retardation of the loss of CO.sub.2 from circulating water can be obtained by addition of less than 50 ppm of quaternary ammonium salts of chloride or bromide. The addition of such salts, however, appears to have no effects on the solubility of CO.sub.2 in water.
Seel, et al. in Angew. Chem., Vol. 67 No. 1. pp 32-33 (1955) report the formation of adducts of SO.sub.2 with anhydrous salts such as (CH.sub.2).sub.4 NF, KF, NaF and RbF.